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Fossil/nodule ID from Eagle Ford formation Dallas, TX


KimTexan

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I went exploring today. I hit 3 places. The 1st two were the Ozan in Rowlett. The first 2 were a busts, but the 3rd was in the Eagle Ford in Dallas and it was a very interesting place. I can’t say that I found specific fossils per se, but I did find the product/remains of prehistoric animals.

I was ecstatic with my finds. Septarian nodules have been on my bucket list of things to find. I found a hill full of them!!! I got there less than an hour before sundown and was thrilled with what I found. 

 

This one is very cool, but I’m not sure what the original creature actually was. The only thing I can think of is that it was an ammonite and maybe the septa became crystal filled, but that is a total guess. The curves on the edge and sides don’t look right, but I’m not sure exactly how these formed in the particular area. It honestly looked like the badlands or something desolate with nothing growing there and was a very fine soft gray shale.

I found a lot of what I think are aragonite crystals at the site as well as some other beautiful crystals. I have never found any crystals in this form before. So I’m thrilled and hope to go back tomorrow if I don’t get paged into work tonight.

 

This is the one I’m very curious about. Maybe some of you may have seen something like this before, but I have not. I think it is super cool though. I’m pretty sure it was once a critter of some sort. I believe it is a septarian nodule, but there are no septarian marks externally like you usually see. When I was washing off other nodules I found, as I was washing, the exterior began to slough off and the septarian lines became visible.

 

There are basically 7 bundles of crystals across this thing. Some are kind of merged.

#1 This one has me most curious of all.

3C54B902-463D-40E8-803A-25DBA37248E3.jpeg.0824dff82509aaba7a6036b7834e3b36.jpeg

#2 other edge. Right edge is encrusted with what I guess could be considered pyrite disease, rusty material mixed with crystals encrusting it. I have found an ammonite before that was rusty with crystals like this.

FF40A8AF-5DD4-48D5-8B94-CFBC05A161DA.jpeg.178da465ac5ce7d7493aee5f376a96ae.jpeg

#3 Close up of the encrustation with small crystals jutting out. They’re hard to see.

FC3736BA-43B4-4282-9846-749B2ED27559.jpeg.38db22ca15657096af3271a1f0e29d5a.jpeg

 

#4 one side. The other side is less descript. The curve on this doesn’t look ammonite to me. I found some pieces that looked almost turtle like, but I don’t know turtle stuff when I see it. I will say the rock material is soft and is a fine shale like texture and material so when wet it becomes slick almost like soap, but no bubbles.

23E295D0-B5B4-4F75-BC94-CB101E15CC6A.jpeg.681efda69491e3ccbe46ae82969d3e56.jpeg

 

This is a 2nd find in the same area. In the center below the crystal on top is a sea shell. Wish I knew what kind, but not much is exposed. All you can see is the mother of pearl inside the shell.

#5 note crystal branching in 3 directions below shell, there is also a beautifully formed crystal on top that is pristine and wafer thin that was part of the septarian, but the non-crystalline material has eroded away leaving the flower petal like crystal.

893BC350-1BDB-4CC7-8BF8-6418E3A0F7D6.jpeg.3be167b6aeae22addda000a856f81ac7.jpeg

#6 This is it from another side. You can see more of the septarian sections. If anyone knows the critter this arose from or the crystal type I’d like to know it. Calcite and gypsum are most common here. There is brown too. I also found stone with green crystals.

1B10A5D8-2298-448C-8268-D0A6E32BAC97.jpeg.b8d112a92bc2e32c67570fffe20b64fb.jpeg

 

This is a 3rd find in the same area.

#7 I found these bars just laying on the ground like this. They looked so peculiar. I couldn’t figure out what they were. I thought they looked a little like columnar basalt, but knew that wasn’t it. There was a small nodule in the ground less than a foot away that was cracked all over and filled with crystal. I wanted to dig it out. In the process of digging it out I found another hard object just under where these were. I thought it was a solid rock, but when I started to move it the rock came apart in these shapes.

96001D34-4C2B-418F-9F1A-5DC9F92A8B60.jpeg.d27946a5a87ce979aa30d3f45a1ea2d7.jpeg

#7 cleaned up a little at home.

DA89EC54-CD3E-4376-837D-36AB130CBBDB.jpeg.4265c9fe31629beeecab1404bc582c12.jpeg

#8 one up close. I believe they are all covered in aragonite or brown crystals. I’ve been told that when an ammonite is encased in a nodule and then the mineralization process takes place that often aragonite crystals form as a result from the nacre in the ammonite shell. I have not found the source of that claim though. Can anyone tell me how these formed and what the crystal is? Is it aragonite? How would I know?

8F494025-99F0-46D2-B650-32FE53AE8E75.jpeg.cba23b4760e0e3f640eb503a8e185233.jpeg

 

 

I can provide higher resolution pics upon request. I had to make these low res to get all of them in here.

@Uncle Siphuncle, @Fruitbat and @BobWill you 3 have hunted the Dallas and North Texas area for many years. I assume you’ve seen these sorts of things and may have insight you can share.

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The first set look like septarian nodule pieces.

Not sure about the other pieces, could be selenite or calcite. Both can fill cracks in the bedrock.

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Congratulations on a bucket list hit.

You may have come across a site that could produce ammonites, along with some other interesting specimens,

but I see none that you have here.

If you have access to a black light, expose them to it, some areas produce fluorescent crystalline material.

Some may fluoresce green while others are a dull orange.

The area around Joe Pool and along the Loop 12 corridor produced these for many years but the years of construction and erosion has taken it's toll.

I know, I was operating heavy equipment (in the 80's) and had first hand grab at some excellent specimens.

There are still some isolated areas that produce.

 

Jess B.

 

 

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What I see here are definitely crystallized septarian sections rather than ammonites.

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1- 6 Septarian concretions / nodules.
7 - 8 You can try the acid test for calcite-aragonite to see if they fizz. Also, I think they could be broken septarian concretion core elements.

 

DA89EC54-CD3E-4376-837D-36AB130CBBDB.jpeg.4265c9fe31629beeecab1404bc582c12.jpeg.b5f3127fa2f79b99d45625155f9b39f9.jpeg8458531.jpg.a4a52e5b645e9235a1b7bda6ea8ef721.jpg comparative picture from here

 

 

 

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@abyssunder

They absolutely are septarian nodule fragments. As I was digging up another nodule I hit something hard right under where the bars lay. It was part of a septarian nodule with a few bars loose, but still in situ with the rest of the nodule and it looked almost exactly like your pic with dirt on it though.

I took this pic. Not as nice and clean though.2979AA57-ADB2-4554-955C-7A19743E0B07.thumb.jpeg.a0412d716b0677b9979740f047d254c5.jpeg

There is dirt covering the side of the nodule, but you can see the bars lose on the top left and right. I had not figured out what they were until I uncovered this.

 

@Ludwigia

I don’t see ammonite either. My mention of ammonites maybe that I have a misunderstanding about how these particular type of septarian nodules form or what the original things they formed around are. I suppose it could have formed around an Inoceramus clam vs an ammonite. The nodules can be enormous. I found some yesterday that must weigh over 300 lbs. There were a couple about 2.5 to 3 feet in diameter and more than a foot thick..

This is a small fragment from the edge of one of them. There is white to clear crystal down the center with brown crystal along the edges.

I could be totally wrong about this, but from personal observation of fossils I have found in North Texas I have seen many clams and ammonites as well as other organisms where the soft tissue appears to have been replaced by white crystal and the shell externally remains.i don’t see the crystals on the outside of the organism.

7FA9FA3E-0DEF-4199-B23C-10F8D65DEAB6.thumb.jpeg.eeb51f23425d8946b6fe055a4b15809d.jpeg

My understanding, from what someone told me, was that the aragonite somehow arises from the nacre in sea shells during the mineralization process. If someone can explain how aragonite forms I truly wish to be enlightened and educated. I’d like to have a correct understanding of the process. So from observation and what I have been told I tend to draw the conclusion that in this example it is possible the white arose from elements in the soft tissue and the brown from elements in the shell. Those are big assumptions and merely anacdotal in a sense. I’d like to be properly educated though with research to support what actually happens.

Maybe @doushantuo has an article that may educate me on the topic of septarian nodule and aragonite formation. 

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one of the authors is Mr. concretions himself,Rob Raiswell

 

 

jjojrhtal.jpg

 

 

 

jjorhtal.jpg

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jjojrhtal.jpg

this flowchart illustrates/ factors in the causative mechanism of concretion formation,but perhaps of any lithological phenomenon as well

 

jjeojrrhtal.jpg

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On 16.1.2018 at 6:29 AM, KimTexan said:

 

@Ludwigia

I don’t see ammonite either. My mention of ammonites maybe that I have a misunderstanding about how these particular type of septarian nodules form or what the original things they formed around are. I suppose it could have formed around an Inoceramus clam vs an ammonite. The nodules can be enormous. I found some yesterday that must weigh over 300 lbs. There were a couple about 2.5 to 3 feet in diameter and more than a foot thick..

This is a small fragment from the edge of one of them. There is white to clear crystal down the center with brown crystal along the edges.

I could be totally wrong about this, but from personal observation of fossils I have found in North Texas I have seen many clams and ammonites as well as other organisms where the soft tissue appears to have been replaced by white crystal and the shell externally remains.i don’t see the crystals on the outside of the organism.

 

Concretions always form around a nucleus, which is almost always organic, although the nucleus could be very small and be dissolved away or transformed into mineral substance quickly, leaving nothing of an organic nature behind. Your samples stem from septarian type concretions. In this case, after the concretion is formed, shrinkage sets in, causing cracks to evolve, which in turn are filled with precipitated mineral substance. The white mineral which you see is calcite. Aragonite is the more unstable form of calcium carbonate which forms the basis of most shells, which is then usually transformed into the more stable form of calcite during diagenesis.

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On 1/16/2018 at 7:29 AM, KimTexan said:

@abyssunder

They absolutely are septarian nodule fragments. As I was digging up another nodule I hit something hard right under where the bars lay. It was part of a septarian nodule with a few bars loose, but still in situ with the rest of the nodule and it looked almost exactly like your pic with dirt on it though.

I took this pic. Not as nice and clean though.2979AA57-ADB2-4554-955C-7A19743E0B07.thumb.jpeg.a0412d716b0677b9979740f047d254c5.jpeg

There is dirt covering the side of the nodule, but you can see the bars lose on the top left and right. I had not figured out what they were until I uncovered this.

 

That is a very nice in situ example of how a septarian nodule is decomposed to its elements due to the weathering. I will use your picture for reference in the future, if you don't mind. Thank you for the rare picture and your interesting topic. :)

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@abyssunder you’re more than welcome to use it, but I thought the example you shared from the Sheppy beach article was quite spectacular. It looked so clean and perfect.

Mine example was buried and it was quite fortunate for me that I found it while digging out this little nodule:

1AB4CBF2-89D0-4148-93C3-88BBE17D3B73.thumb.jpeg.bee60c5a1144b72567b1fa3925fe7a19.jpeg

It looks like a “baked potato” so that’s what I call it. I tried digging it out, but it still just crumbled. I removed the biggest part and saw this:

4DCF219F-9949-4BDF-A3AA-D5B999D272AA.thumb.jpeg.aa4289905dffde8ce001c27c213585f7.jpeg

The entire nodule with the exception of the thin crust had crystallized. All the white and yellow is crystal. Most of the thin crust was brown crystal.

I say fortunate, because it was extremely informative and educational to see it in situ like that. I took one geology class about 25 yrs ago after I had graduated from college. We didn’t get much field experience. Although we did make a trip to Big Bend National Park, which was a great trip for geology.

The picture of the septarian nodule with bars was about 3-4 inches to the left of the “baked potato” nodule.

I think I found 6-8 nodules that day in maybe a 40 foot long area, all pretty cool looking. I returned to the area the next day to retrieve a larger nodule that was too heavy for me to carry along with all the other ones I found. The fragment was 12” long x 6” inches think. I also visited another area a few miles away and found what I think is a spectacular fragment 13 x 6 inches. There were also 5 or more complete nodules from about 2 to maybe 2.5 feet in diameter. I’m sure some must weigh over 200 lbs. I wish I had the means to retrieve them and bring them home. Unfortunately I didn’t take any pics, because I had dropped my phone on he walk out without realizing it, but thankfully found it on the return.

 

 

@Ludwigia I was familiar with everything stated except your very last statement that aragonite was a less stable form of calcium carbonate. It makes sense. I was not aware that aragonite could be transformed into calcite. So that was informative and new to me.

My knowledge of minerals/crystals is pretty limited. I think I have read that the conversion only happens by dissolution of the aragonite first though.

I agree the explaination you gave about how they form may easily apply to the nodule in picture #1, but I don’t believe the explaination would satisfactorily explain how the nodule pictured in #5 and #6 formed since it has a pattern similar to the  reference kindly posted by @doushantuo  above of the almost perfect pentagonal shapes. 

I suppose you could have the nodule form, the organism then decomposes and leaves a vug or hollow and then you have crystal formation freely occur without structural restraints. 

 

I found this article that sounds like a good reference for this topic, but I have only read the abstract. It covers the process of conversion of aragonite to calcite from examples in Texas.

 

https://pubs.geoscienceworld.org/sepm/jsedres/article-abstract/36/3/733/95885/processes-of-conversion-of-aragonite-to-calcite#.Wl_xxMLezKY.email

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Very nice finds to check off that bucket list of yours.

Thanks to the other members who post information on how they form.

I learn more and more from all of you who post up pic

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rtal.jpg

Something i consider worthwhile mentioning,form Buchardt & Weiner's 1991 study of aragonite in Cretaceous ammonites

bujjeojrrhtal.jpg

quggderosteollptttrymjjpwillist.jpg

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